1. Field of the Invention
The present invention relates to a tone control device and, more particularly, to a separated or built-in type tone control device for audio equipment providing with a frequency characteristic compensating function as well as a bass-treble type tone control function within the audio frequency range.
2. Description of the Prior Art
There are two major objects in the tone control function of prior art audio equipment.
As for the first object, if the played-back signal from a recording medium is converted into sound without making any compensation, the reproduced sound lacks fidelity to the original sound because the frequency characteristic of on the medium is not linear. Therefore, because of this nonline frequency transfer characteristic of media such as that used in recording, the transmission and the transformation must be compensated in order to reproduce a sound with fidelity to the original sound. The compensation has generally been made, for example, to the frequency transfer characteristic in a speaker or an acoustic space. In this method of compensation, the gain of finely divided frequency bands are independently controlled for attaining a flat frequency characteristic. A so called graphic equalizer is a known device for achieving this first object. In the graphic equalizer, the audio frequency band is normally divided into three or more bands and the gain of each band is controlled independently for the required compensation.
The second object is generally achieved through an artificial tone control to change the frequency characteristic for making low frequencies more abundant in sound or for making the sound with stimulating tone more comfortable to listen to. This type of tone control normally does not require complicated control of the frequency characteristic and is provided by a bass/treble type tone control circuit wherein the audio frequency range is divided roughly into two frequency bands of higher and lower parts and the control is done separately for the lower and higher frequency bands as they are defined as bass and treble control respectively.
In FIG. 1, there is shown a circuit diagram of a prior art graphic equalizer, wherein OP1 denotes an operational amplifier, R1 is a resistor connected between an input terminal T1 and a non-inverting input terminal (hereinafter called "+" input terminal) of the operational amplifier OP1, VR1-VR5 are variable resistors connected in parallel to the "+" input terminal and an inverting input terminal (hereinafter called "-" input terminal) of the operational amplifier OP1 with their fixed resistance arms, and RC1-RC5 are series resonance circuits each of which consists of a coil, a capacitor and a resistor connected in series and connected between slide arms of the variable resistors VR1-VR5, respectively, and the ground. The series resonance circuits RC1-RC5 have resonance frequencies of 100 Hz, 316 Hz, 1 KHz, 3.16 KHz, and 10 KHz, respectively. Further in the FIG. 1, R2 is a feed back resistor connected between the "-" input terminal of the operational amplifier OP1 and an output terminal T2.
In accordance with this prior art arrangement, by applying an audio signal to the input terminal T1 and by controlling each of the variable resistors VR1-VR5, the gains of the five frequency bands which have the center frequencies of 100 Hz, 316 Hz, 1 KHz, 3.16 KHz, and 10 KHz respectively are varied and there is derived an audio signal having the frequency characteristic which conforms to the varied gains of the frequency bands from the output terminal T2 of the operational amplifier OP1. In this case, if the variable resistors are controlled in such a manner that the resistance at the "-" input terminal side of the operational amplifier OP1 becomes smaller in value as compared to the resistance at the "-" input terminal side, the gain for the corresponding frequency band is increased. Similarly, the gain is decreased if the direction of the control is reversed.
FIG. 3 is a diagram showing the change of frequency characteristics of the graphic equalizer of the type shown in FIG. 1, wherein the gain for each frequency band is varied between the maximum (12 dB) and the minimum (-12 dB) at the center frequency of each band.
FIG. 2 is a circuit diagram showing a conventional bass/treble type tone control circuit of the prior art. This type of tone control circuit consists of an input terminal T3, an output terminal T4, an operational amplifier OP2, a pair of resistors R3, R4, three capacitors C1-C3, a variable resistor VR6 for bass control, and a variable resistor VR7 for treble control.
In the arrangement above, the gain of a low frequency band of an audio input signal fed through the input terminal T3 is controlled by adjusting the variable resistor VR6 for bass control, and the gain of a higher frequency band thereof is controlled by adjusting the variable resistor VR7 for treble control, so that an audio output signal whose tone corresponds to these gains is produced at the output terminal T4 of the operational amplifier OP2.
FIG. 4 is a diagram showing the change of typical frequency characteristics of the bass/treble type tone control circuit of the type shown in FIG. 2, wherein the gains of the high frequency band as well as the low frequency band are adjustable from the maximum of 12 dB to the minimum of -12 dB at the upper and lower sides of 1 KHz.
There is another type of prior art graphic equalizer different from the one shown in FIG. 1 in that electronic variable resistors are utilized instead of the variable resistors VR1-VR5. A circuit diagram of such a graphic equalizer having electronic variable resistors is shown in FIG. 5, wherein numeral 1 denotes a manipulation input section which is externally manipulative via keyboard entry to increase (UP) or decrease (DOWN) the gain level of each of the five bands having the center frequency of 100 Hz, 316 Hz, 1 KHz, 3.16 KHz, and 10 KHz respectively. Numeral 2 denotes a digital processing unit of the type known as a microcomputer in the art, and numeral 3 denotes an electronic variable resistor assembly provided for each of the five bands and the resistance of which is varied in accordance with a command from the microcomputer 2. The electronic variable resistor assembly 3 consists of an interface circuit 4 and an electronic variable resistor unit 5. The electronic variable resistor unit 5 further includes an array of resistors and an array of electronic switches each connected to every junction of the array of resistors for being switched by outputs of the interface circuit 4.
As for the UP/DOWN keyboard input at the manipulation input section 1, one valid input may vary the gain by amount of +2 dB or -2 dB in the respective band. The digital processing unit 2 scans the manipulation input section 1 for checking the existence of keyboard operation at all times. Upon detecting the key board operation of an UP key (or DOWN key) assigned to a selected band, the digital processing unit 2 sends out new gain setting data to the interface unit 4 in accordance with a protocol, which turns on electronic switches that correspond to an amount of variation required in the electronic variable resistor assembly 5 after adding +2 dB (or -2 dB) to the current gain setting data in a memory for the selected band. It is obvious that the electronic switches which have been turned ON should be cleared to turn OFF prior to being turned ON in the above mentioned operation. The digital processing unit 2 is so arranged as to invalidate keyboard input that causes the change of gain to be over +12 dB or under -12 dB.
As a characteristic of the graphic equalizer, if the number of divided frequency bands is increased, the frequency characteristic may be made flatter after compensating. However, more keyboard input operations would also be required for increasing or decreasing the gain across wide frequency bands for tone control.
On the other hand, in a bass/treble tone control circuit that requires only two frequency bands in the audio frequency range, very quick input operation is possible because only a few manipulations at the input section are required for tone control. In addition, owing to the fact that the gain is varied across wider frequency bands for tone control, there is a high probability of having strong spectra of voice, music and the like within these frequency bands. Therefore the effect of changing an amount of input at the input section can easily be detected by monitoring the sounds during the input operation, resulting in very few errors due to misjudgment in the determination of the desired amount of input as compared to the graphic equalizer. For compensation of dips or peaks in the frequency characteristic of speakers, however, the satisfactory compensation is not attained most of time with this type of tone control.
Accordingly, the use of finely divided frequency bands makes manipulation more difficult and the use of coarsely divided frequency bands results in lessened ability to control compensation of frequency characteristic in the prior art tone control devices.
In car audio equipment, it is especially desirable to have a high degree of compensating ability due to the peculiar acoustic environment while permitting control through the the simplest manipulation, due to safety concerns. In a tone control device to satisfy these problems, it is desirable to furnish a device having a tone compensating capacity for multi-frequency bands as in the graphic equalizer, together with simple tone control manipulation means such as in the bass/treble tone control circuit. In tone control devices of this type, there have been such problems that the manipulation input section has been complicated by the installation of switches and levers which are not necessary at all times and the scale of circuitry has been enlarged with the installation of two types of circuits for the tone control. Further, the operation of the manipulative input section of the graphic equalizer with the intent of compensation necessitates brining the frequency characteristic being set to a favorable tone by the bass/treble tone control manipulation back to the flat characteristic causing complicated problems in the operation.
It is, therefore, an object of this invention to solve the above-mentioned problems and to provide a tone control device capable of performing the combined functions of fine compensation for frequency characteristic as in the graphic equalizer and simple manipulation for tone control as in the bass/treble tone control circuit by making use of corresponding circuits in common to both without increasing the scale of circuitry.